Compressed natural gas (CNG) is seeing increased use as a vehicle fuel in the United States and worldwide. CNG can also be utilized for heating and other uses where natural gas pipelines do not exist. Large quantities of biogas (landfill gas, digester gas, anaerobic wastewater treatment process gas and other biogases) are currently being flared to the atmosphere. Biogas is flared when it cannot be utilized economically for on-site power production or where other traditional markets for biogas do not exist. It would be good for both the environment and the economy to convert combustible gases now being flared to a useful form of energy.
The current invention converts biogas to a CNG equivalent. It differs from and is an improvement to the Borray U.S. Pat. No. 5,727,903 for conversion of landfill gas into a CNG equivalent in that the current invention employs a simpler, less costly process chain. The process of the current invention can be scaled down more practically and economically to much smaller sized processing plants, allowing smaller sources of biogas to be exploited.
Biogas, when converted into a natural gas equivalent, and then to a CNG equivalent or a liquefied natural gas (LNG) equivalent, is considered to be what is known as biomethane. Biomethane is a renewable fuel and can be used by producers and users of fossil vehicle fuels to meet renewable fuel utilization goals and/or greenhouse gas reduction goals. Biomethane CNG (B-CNG) commands a premium both environmentally and economically in the vehicle fuel market.
CNG vehicle fuel standards have been established by the California Air Resources Board (CARB) and others. Biogas does not comply with many of the line item specifications in these standards. The methane content of biogas is too low. The carbon dioxide, and sometimes the nitrogen content, of biogas is too high. The moisture content of biogas is too high. The sulfur content of biogas is too high. While compliant with the specifications governing volatile organic compounds (VOCs), certain biogases contain individual VOCs which must be removed (despite not being specifically listed in CNG vehicle fuel specification standards), such as siloxane compounds, as these compounds are known by those in the biogas industry to be harmful to reciprocating engines.
Converting biogas to a CNG equivalent requires: 1) limiting nitrogen at the source of the biogas; 2) dehydration; 3) removal of carbon dioxide; 3) removal of VOCs; 4) removal of sulfur compounds; and 5) compression.